(a) Field of the Invention
The present invention is concerned with an apparatus and a method for separating ions of a pulse by their mass as said pulsed ions are guided along a course. Each of the pulsed ions has a mass m within a range m.sub.min to m.sub.max, a same energy E and a speed v given by v=(2E/m).sup.1/2.
(b) Brief Description of the Related Art
It is known to analyse a given sample by bombarding it with an incident beam (e.g. laser, electron, fast atom or ion beam). The incident beam impacts the sample and the resulting ions, representative of the composition of the sample, are then analysed by a variety of apparatuses such as a time of flight mass spectrometer (TOFMS). In a TOFMS, the composition of the sample can be determined by analysing the masses of the ions resulting from the bombardment. The resolution of a TOFMS is dependent on its length, the speed of the ions, the duration of the pulse and the ability of the apparatus to maintain isochronicity of the travel of ions having the same mass but a slight energy difference.
Known in the art is U.S. Pat. No. 5,140,158 (Richard F. Post), which relates to an improved method and apparatus for separating ions of chosen charge-to-mass ratios from other ions with a different charge-to-mass ratio. There is illustrated a preferred embodiment of the invention showing a cross-sectional view from the side of a single module of the invention. A vacuum chamber contains an array of conducting rods supported on insulated electrical feed-throughs. At a first end of the array are located a pair of parallel plate electrodes and a collector cup which together comprise the collector assembly. At a second end of the array is located an ion source and accelerator. The apparatus induces a series of localized electrical potentials which simulate a travelling electrical potential hill travelling at a velocity with a magnitude v.sub.o. Ions with a charge-to-mass ratio Z/M&gt;k are accelerated to a velocity twice the velocity of the travelling electric potential hill while ions with a charge-to-ratio Z/M&lt;k are not accelerated. Therefore when a travelling potential hill is applied, an ion or charge particle beam is created of ions or charge particles with a charge-to-mass ratio greater than k.
Also known in the art is U.S. Pat. No. 4,912,327 (Allen R. WAUGH) which describes a method and apparatus for producing a pulsed microfocused ion beam. The method comprises the step of deflecting the continuous ion beam by the synchronised actions of a first electric field component Ey, directed along or parallel to a y-axis, and a second electric field component Ex, directed along or parallel to an x-axis. The first electric field component Ey may be generated by applying a periodically-varying voltage waveform Vya to a first y-deflecting electrode, and a periodically-varying voltage waveform Vyb to a second y-deflecting electrode.
Also known in the art is U.S. Pat. No. 5,136,161 (Charles H. LOGAN) which describes a mass spectrometer. As a plurality of ionized particles traverse the ion current path defined by a plurality of drift tubes, a selected portion of the ionized particles will reach the first field region A at the same time that the field generated therein reaches its maximum value. These particles will receive an energy increase, and corresponding increase in velocity, that is greater than that received by ionized particles reaching the first field region A at a time when the electric field is at a magnitude less than its maximum value. Since the increase in velocity is dependent upon the mass of the ionized particle and the amount of energy added to the ionized particle, and since the mass of the synchronous particle is known, the increase in velocity for the synchronous particle is determinable.
Also known in the art there is the article entitled "Mass-Spectrometer With Ion Multiple Passage Of A Magnetic Field" published in Nuclear Physics Institute, Academy of Sciences of Republic of Kazakhstan by S. P. Karetskaya et al. This article describes the construction and the testing of a statical mass-spectrometer in which ions traverse three times a field created by a magnet, having poles shaped as regular hexagons.
Also known in the art is U.S. Pat. No. 4,458,149 (M. Luis MUGA) which describes a mass spectrometer. This invention comprises the steps of applying a time-dependent and time-varying force field to already partially separate iso-mass ion packets along their flight path.
Also known in the art are U.S. Pat. No. 4,238,678 (B. Wayne CASTLEMAN), U.S. Pat. No. 3,397,311 (J. M. SAARI et al.) and U.S. Pat. No. 5,180,914 (Stephen D. DAVIS) which describe methods and apparatuses wherein a static voltage is applied to ions.
Also known in the art are the following U.S. patents which describe different apparatuses and methods involving ion beams: U.S. Pat. Nos. 4,335,465; 4,904,872; 5,065,018; 5,162,649; 5,164,592; 5,196,708; 5,371,366; 5,431,714; and 5,463,220.
Additionally, T.Sakurai and M.Baril, in Nuclear Instruments & Methods (vol. 369, pp.473-476, 1995), have proposed a theoretical model of a closed circuit mass spectrometer. It uses electrostatic ion mirrors and a centered magnetic prism. The ions must come to a stop in an ion mirror and reflect in the reverse direction. This is troublesome for keeping the ions in the closed circuit for prolonged periods of time. In addition, it is not properly a time of flight device but a multiple pass analyser.
Also known is the work of Ching-Shen Su, in International Journal of Mass Spectrometry and Ion Processes (vol. 88, pp. 21-28, 1989), describing a time of flight mass spectrometer implying a fixed number of reflections of an ion pulse between two sets of electrostatic planes. This system does not include a closed circuit path. There is no ion focussing hence there is substantial loss of beam intensity for each reflection. For the prototype described, the maximum number of reflections achieved was 4, giving a mass resolution of only about 300 (at base peak) around mass 85.
Also known is the work of H. Wollnik and M. Przewloka, in International Journal of Mass Spectrometry and Ion Processes (vol. 96, pp.267-274, 1990), describing a system similar to the preceding one but in a folded geometry. It uses 1 permanent ion mirror and 2 switchable ion mirrors positioned to form a V path. When activated, the switchable ion mirrors hide the pulsed ion source at one end and the detector at the other end. The use of ion mirrors is troublesome for keeping the ions in the closed circuit for prolonged periods of time. There is no ion focussing nor confinement in this system hence there is substantial loss of beam intensity for each reflection. For the prototype described, the maximum number of reflections was 5, giving a mass resolution of only 720 around mass 28.
Also known is the work of Trotscher et al., in Nuclear Instruments & Methods (vol. B70, pp. 455-458, 1992), describing a hexagonal magnetic storage ring for high energy ions. This setup is of sizable dimensions (40 meters in width) and measures mass by frequency and not by time of flight. The magnetic sectors preserve momentum and not energy so that only one very precise mass gets to be stored for a certain number of turns. Also, the insertion of ions in the closed path is very difficult because it must be done across a magnetic field and subsequently corrected in the path: consequently, the insertion efficiency is of the order of 0.5% only.
There is finally known in the art, the published work of Wollnik, H. "Energy-Isochronous Time-of-Flight Mass Analyzers". This paper discusses techniques of time-of-flight mass analyzers, both for systems of reflector-type geometry and systems that employ sector fields. Wollnik does not however provide a system that allows to separate pulsed ions by mass as the pulsed ions are guided along a course in a simple, inexpensive and efficient manner while providing a variable resolution, even in a case where the ions have masses which are close to one another.
None of the above-mentioned patents or published works shows or describes the necessary means for separating pulsed ions by mass as said pulsed ions are guided and confined along a purely electrostatic course in a simple, inexpensive and efficient manner while providing a variable resolution. This is especially true if one wants to separate ions having very close masses.